In a three-phase system, there are some typical characteristics to consider, especially when dealing with an unbalanced load. Let's evaluate the given options one by one:

1. The sum of phase angles of each phase is always 120°.

   This statement is generally true for a balanced system, where the phase separation is uniformly 120°. However, an unbalanced load can cause deviations in angles due to unequal impedance, rendering this statement false for an unbalanced system.

2. In an unbalanced three-phase system, the neutral current is zero.

   This is incorrect. In an unbalanced system, the currents in each phase are not equal, which can cause a net current to flow through the neutral conductor.

3. The sum of the line currents is not necessarily zero.

   This statement is true. In an unbalanced system, the line currents can differ significantly and do not necessarily sum to zero, causing current to flow through the neutral.

4. The line voltages are always equal.

   This statement is problematic. In an unbalanced three-phase system, it is possible for line voltages to be unequal due to differences in the load impedance across the phases. Thus, while line voltages might be designed to be equal, they won't necessarily remain so under unbalanced conditions.

After evaluating the options, it becomes clear that the option indicating that "the sum of the line currents is not necessarily zero" accurately describes a typical condition of unbalanced three-phase systems. Thus, this is the most accurate statement concerning unbalanced loads.